1. Field of the Invention
The present invention relates to wavelength division multiplexing (WDM) optical transmission equipment with redundant configuration, and a wavelength control method of light output in the standby system of the optical transmission equipment.
2. Description of the Related Art
An optical transmission system employing a wavelength division multiplexing (WDM) transmission technique is used for large-capacity communication systems.
The optical transmission system is constituted of WDM optical transmission equipment sets interconnected by optical transmission lines. The WDM optical transmission equipment is constituted of a plurality of optical transmitters outputting optical wavelength signals modulated by the signals from signal sources, and a multiplexer multiplexing the optical wavelength signals having different wavelengths output from the plurality of optical transmitters. The multiplexed signal is then forwarded to a relevant optical transmission line.
To obtain stable operation of the optical transmission system, each set of the optical transmission equipment has a redundant configuration, namely, an active system and a standby system. FIG. 1 is an exemplary configuration of the WDM optical transmission equipment in the optical transmission system.
The plurality of optical transmitters SD1-SD4 respectively outputs optical signals of different optical wavelengths λ1-λ4. The optical signals of the optical wavelengths λ1-λ4 output from the plurality of the optical transmitters SD1-SD4 are input to corresponding input ports of a multiplexer 100 (exemplarily structured of an arrayed waveguide grating: AWG), in which the optical signals are wavelength-division-multiplexed. The wavelength-division-multiplexed output is input to an optical amplifier 101, in which the output level therefrom is controlled constant, and amplified. Then the amplified output is forwarded to a non-illustrated optical transmission line.
The plurality of optical transmitters SD1-SD4 are of identical structure. In the lower part of FIG. 1, an enlarged configuration of the optical transmitter SD4 is shown, as one example.
The optical transmitter SD4 has an active system and a standby system, each including an optical transmission circuit 1a (active system), 1b (standby system) having a laser diode of which emission wavelength is controllable (which is termed ‘tunable LD’) with an electric/optical conversion function, and a variable optical attenuator (VOA) 2a (active system), 2b (standby system) for controlling an attenuation amount against each output. The outputs of variable optical attenuators (VOAs) 2a, 2b are input to a coupler 3, combined and output therefrom.
Here, during operation of the active system, it is necessary to prepare a standby panel and set therein, so that switching to the standby system becomes ready.
Each of the active panel and the standby panel includes corresponding optical transmission circuit 1a, 1b and attenuator 2a, 2b for attenuating the output of optical transmission circuit 1a, 1b. 
When resetting the panel on the standby side while the active side is in operation, in order to set the wavelength of the tunable LD into the optical transmission circuit on the standby side to be switched to, and to perform bias setting into the modulator for modulating the wavelength of the tunable LD, it is necessary to emit light once on the standby side.
At this time, in the conventional configuration shown in FIG. 1, optical signals having an identical wavelength λ4 are output from optical transmission circuits 1a, 1b on the active side and the standby side, respectively.
In this case, it is necessary to set the attenuation amount in variable optical attenuator (VOA) 2b as maximum against the output of optical transmission circuit 1b on the standby side.
There are broadly two reasons for setting variable optical attenuator (VOA) 2b to the maximum attenuation:
First, in the optical transmitter of the panel on the standby side to be reset, unless the light level input from the standby system to coupler 3 is controlled as small as possible, an optical signal in operation, which is input to coupler 3 from optical transmission circuit 1a and variable optical attenuator (VOA) 2a on the active side, is coupled with the light from the standby system having the identical wavelength. This affects the optical signal in the operation system, and causes deteriorated transmission quality.
Secondly, in FIG. 1, the output of multiplexer 100 is input to optical amplifier 101 in which the output light level is controlled constant by an automatic level control (ALC) function.
More specifically, the optical signals from optical transmission circuits 1a, 1b have the identical light wavelengths λ4, and therefore if light is also output from optical transmission circuit 1b in the standby system, the output level of coupler 3 becomes large. Then, because of the ALC function provided in optical amplifier 101, the levels of the components from the other optical transmitters SD1-SD3 are controlled relatively small.
Now, in FIG. 2, detailed configurations of an optical transmission circuit 1a (1b) and a variable optical attenuator (VOA) 2a (2b) are shown.
In optical transmission circuit 1a (1b), the controllable emission wavelength from the laser diode (tunable LD: shown as TN-LD in the figure) 10 is controlled by wavelength controller 11, and the wavelength controlled light output from the tunable LD is output after being modulated in modulator 12 correspondingly to a transmission signal. A bias controller 13 controls a bias voltage against modulator 12, in proportion to the signal level.
Meanwhile, variable optical attenuator (VOA) 2a (2b) includes a current-variable attenuator 20, producing a predetermined attenuation by supplying a reference current from a drive circuit 22. Further, a photodetector 21 senses the output level of coupler 3, and drive circuit 22 controls the drive current based on the sensed output level. In such away, the attenuation amount of attenuator 20 is controlled constant.
Here, current-variable attenuator 20 has a temperature dependant attenuation property as shown in FIG. 3. In FIG. 3, the horizontal axis represents a current fed from drive circuit 22, while the vertical axis represents a loss amount, i.e. attenuation. A maximum attenuation is obtained at a predetermined current value.
As shown in the figure, the attenuation property varies with the temperature, and therefore the control amount (drive current) to obtain the maximum attenuation also varies with the temperature. As explained earlier, when resetting the standby system, it is necessary to produce the maximum attenuation against the output of optical transmission circuit 1b in the standby system state. However, because the attenuation property varies with the temperature, the precise attenuation property cannot be known, and accordingly, it is difficult to set the drive current which produces the maximum attenuation.
Therefore, in this case, the outputs of other optical transmitters are affected, as explained before.
Here, in the official gazette of the Japanese Unexamined Patent Publication No. 2003-298524, there is disclosed an invention in respect to a startup control method of a laser diode in the WDM technique. However, this invention is aimed to prevent deterioration caused by crosstalk with an adjacent wavelength when starting up the laser diode. There has been no suggestion about solving the difficulty in setting the standby system during operation in the active system.